1. Field of the Invention
This invention relates to a superconducting cermet material formed from a triple-layer perovskite type superconducting ceramic material and one or more precious metals or alloys thereof, or metals or alloys thereof more noble than copper; and a method of making the superconducting cermet.
2 Description of the Related Art
Since the discovery of superconductivity in 1911, the phenomena of a material being able to conduct electricity with almost no resistance when the material is cooled to a temperature approaching absolute zero (0.degree. K.) has remained an interesting scientific curiosity having few applications which would justify the expense of maintaining the necessary liquid helium cooled system.
Recently, however, superconducting ceramic materials have been produced which exhibit this phenomena at much higher temperatures, e.g., the boiling point of liquid nitrogen which boils at about 77.degree. K. The ability to produce superconductivity in a material cooled by liquid nitrogen completely changes the economics which have previously restricted the applications to which superconductivity could be applied.
These new ceramic materials are sometimes referred to as triple-layer perovskite compounds because of the crystallography of the resulting structure; or 1-2-3 compounds because of the atomic ratios of 1 atom of a rare-earth element such as in the Lanathanide series (La, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) or yttrium; 2 atoms of an alkaline earth metal such as barium or strontium; and 3 atoms of copper. The superconducting ceramic also contains from 6.5+ to 7- atoms of oxygen which is usually referred to as O.sub.(6.5+x) where x is greater than 0 and less than 0.5, resulting in a chemical formula such as, for example, YBa.sub.2 Cu.sub.3 O.sub.(6.5+x).
Saito et al in an article entitled "Composition Dependence of Superconductivity in Y-Ba-(Ag, Cu)-O System" in the Japanese Journal of Applied Physics. Vol. 26, No. 5, May, 1987, pp. L 832-833, (incorporated herein by reference) have substituted silver for copper in the superconducting ceramic and found that as the level of silver was increased, superconducting capability decreased with temperature, particularly at higher levels of silver. While the superconducting properties of such a ceramic material have been confirmed by demonstration of the Meissner effect wherein the superconductor, when cooled to superconducting temperature, will exhibit magnetic properties sufficient to levitate a magnet above the superconductor, the material is deficient in some of the essential physical properties needed to permit fabrication and practical usage of structures from the material.
Most notable of these deficiencies is the extreme brittleness and poor mechanical strength of the superconducting ceramic structures which inhibits formation of shaped structures, e.g., coils or wires therefrom; and the low current carrying capabilities of the superconducting ceramic. The superconducting ceramic material also shows evidence of microcracking which is a further indication of its brittleness and would also effect its transport critical current density J.sub.c (at zero magnetic field). Most applications of the new high T.sub.c superconductors require high critical current densities (J.sub.c) of more than 10.sup.4 -10.sup.5 A/cm.sup.2, often in the presence of a significant magnetic field (1-10 Tesla). Critical current density J.sub.c in bulk sintered samples of YBa.sub.2 Cu.sub.3 O.sub.6.5+x are typically 10.sup.2 -10.sup.3 A/cm.sup.2 only. In a magnetic field, these values are even lower. Thus there is great need for improved critical properties (see "Transport Critical Current in Bulk Sintered Y.sub.l Ba.sub.2 Cu.sub.3 O.sub.x and Possibilities for its Enhancement", J.W. Ekin, Advanced Ceramic Materials, Vol. 2, No. 3B, Special Issue, 1987, incorporated herein by reference). In addition, the superconducting ceramic material, as produced, is of rather low density, i.e., about 60-75% of theoretical density, and is difficult to densify, resulting in a low environmental stability
and a sensitivity to moisture and CO.sub.2. Low density also leads to poor superconducting and mechanical properties.
It would, therefore, be of great value to be able to produce a compound or structure from such ceramic superconducting material which would, while preserving the superconducting effects of the ceramic, have superior mechanical strength, exhibit less brittleness (i.e., be more malleable), improve critical current density capabilities, reduce or eliminate microcracking, and have a higher density, as well as improving the environmental durability of the material.
In parent applications Ray Ser. No. 085,959, entitled "Liquid Phase Sintered Superconducting Cermet"; Ray Ser. No. 086,024, entitled "Polycrystalline Superconducting Cermet"; and Ray Ser. No. 086,025, entitled "Superconducting Cermet"; all filed on Aug. 14, 1987 and all assigned to the assignee of this invention, there are disclosed and claimed superconducting cermets, and methods of making same, by sintering together a mixture of a superconducting ceramic and one or more noble metals or metal alloys thereof The noble metal, or alloys thereof, sintered with the superconducting ceramic preferably comprise one or more noble metals, e.g., gold, silver, and the six platinum metals, or alloys which include same, while the superconducting ceramic is, generally, of the triple layer perovskite or 1-2-3 type compounds.
However, in those applications, it is generally contemplated that the superconducting ceramic material will be first formed and then the superconducting ceramic material will be mixed with the noble metal and the mixture then sintered